Offset-gate-type semiconductor device

ABSTRACT

A semiconductor device includes a first semiconductor region having a first conductivity type, a second semiconductor region formed on the first semiconductor region and having the first conductivity type, a third semiconductor region formed in a surface of the second semiconductor region and having a second conductivity type, a fourth semiconductor region formed in the surface of the second semiconductor region and having the second conductivity type, and a gate structure formed on the second and fourth semiconductor region. The semiconductor device further includes a conductive member arranged in the trench extending from a surface of the fourth semiconductor region to the first semiconductor region, the trench having one sidewall surface flush with a sidewall surface of the gate structure.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Applications No. 2000-380094, filed Dec. 14,2000; and No. 2001-283974, filed Sep. 18, 2001; and U.S. patentapplication Ser. No. 10/013,874, filed Dec. 13, 2001 now U.S. Pat. No.6,552,180, and U.S. patent application Ser. No. 10/388,509, the entirecontents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device and, moreparticularly, to a semiconductor device for synchronous commutation,which executes high-frequency operation.

2. Description of the Related Art

A low-ON-resistance characteristic has conventionally been regarded asimportant for a DC/DC synchronous buck converter used in a computer orthe like to improve the efficiency. For this reason, a trench-gate-typeMOS transistor has widely been used to form a DC/DC converter. However,along with the recent rise in operation frequency, not only a low ONresistance but also a decrease in switching loss is required. Hence, itis important for a MOS transistor of a DC/DC converter to decrease notonly the resistance but also the capacitance. From this viewpoint, atrench-gate-type MOS transistor is not preferable because of itsstructure in which a gate electrode opposes a drain layer via a thingate insulating film. With this structure, the area can hardly bereduced, and therefore, the parasitic capacitance between the gate andthe drain is large.

In place of a trench-gate-type MOS transistor, an offset-gate-type MOStransistor has begun to be used to form a DC/DC converter. The structureof a conventional offset-gate-type MOS transistor usable for a DC/DCconverter has been proposed in, e.g., Malay Trivedi et al., “Comparisonof RF Performance of Vertical and Lateral DMOSFET”, ISPSD99,Proceedings, pp. 245–248. Jpn. Pat. Appln. KOKAI Publication No.5-121739 also discloses an insulated gate semiconductor device. As anexample, the structure proposed by Malay et al. is shown in FIG. 1. FIG.1 is a sectional view of a MOS transistor.

As shown in FIG. 1, an n⁺-type source region 12, n-type LDD region 13,n⁺-type drain region 14, and p-type body region 15 are formed in thesurface region of a p⁻-type epitaxially grown layer 11 on a p⁺-typesubstrate 10. A gate electrode 16 is formed on the body region 15between the source region 12 and the LDD region 13. A source electrode17 is formed on the source region and body region 15. A drain electrode18 is formed on the drain region 14. A reach through layer 19 is formedto connect the source electrode 17 and substrate 10. A source electrode20 is formed on the lower surface of the substrate 10.

According to the above-described structure, the source electrode 20 canbe formed on the lower surface of the substrate 10 by preparing thereach through layer 19. For this reason, the parasitic capacitance orparasitic inductance of the MOS transistor can be reduced. As aconsequence, the MOS transistor can have a low resistance and can beoperated at a high frequency.

However, the reach through layer 19 is most generally formed by impuritydiffusion. Hence, the width of the reach through layer 19 is inevitablyrelatively large. In some cases, the reach through layer 19 occupiesabout ½ the area of the entire MOS transistor. For this reason, when thereach through layer 19 is formed, the size of the MOS transistor becomeslarge.

In actual manufacturing, the gate electrode 16 and source electrode 17must be separated by a relatively large distance. This is because themisalignment of masks to be used to form the gate and source electrodesis taken into consideration. Then, the width of the body region 15immediately under the source region 12 increases. Hence, the resistancevalue of the p-type body region with respect to holes becomes large, andthe ruggedness to avalanche current of the MOS transistor deteriorates.

Additionally, the drain interconnection layer is normally located on thegate electrode 16. The gate electrode 16 is adjacent to the drainelectrode 18 in the horizontal direction and to the draininterconnection layer in the vertical direction. As a result, thefeedback capacitance of the MOS transistor increases.

BRIEF SUMMARY OF THE INVENTION

A semiconductor device according to an aspect of the present inventioncomprises:

a first semiconductor region having a first conductivity type;

a second semiconductor region formed on the first semiconductor regionand having the first conductivity type and a resistance higher than thefirst semiconductor region;

a conductive member arranged in a trench formed in the secondsemiconductor region and having a depth from a surface of the secondsemiconductor region to the first semiconductor region;

a third semiconductor region formed in the surface of the secondsemiconductor region while being separated from the conductive memberand having a second conductivity type;

a fourth semiconductor region formed in the surface of the secondsemiconductor region in a region between the conductive member and thethird semiconductor region and having the second conductivity type, thefourth semiconductor region being in contact with the conductive memberand being separated from the third semiconductor region; and

a gate structure formed on the fourth semiconductor region and on thesecond semiconductor region between the third and fourth semiconductorregions, the gate structure having one sidewall surface flush with asidewall surface of the trench.

A semiconductor device according to other aspect of the presentinvention comprises:

a first semiconductor region having a first conductivity type;

second and third semiconductor regions formed in a surface of the firstsemiconductor region and having a second conductivity type, the secondand third semiconductor regions being separated from each other;

a gate insulating film formed on the first semiconductor region betweenthe second and third semiconductor regions;

a gate electrode formed on the gate insulating film;

first and second electrodes formed on the second and third semiconductorregions, respectively;

a dielectric interlayer formed on the first semiconductor region tocover at least the second electrode and gate electrode;

an interconnection layer formed on the dielectric interlayer immediatelyabove at least the gate electrode and electrically connected to thefirst electrode; and

a first conductive film formed in the dielectric interlayer immediatelyabove the gate electrode while being separated from the gate electrodeand interconnection layer and having the same potential as that of thesecond electrode or a predetermined potential between the first andsecond electrodes.

Further, a method for fabricating a semiconductor device according toother aspect of the present invention comprises:

forming a second semiconductor region having a first conductivity typeand a resistance higher than a first semiconductor region on the firstsemiconductor region having the first conductivity type;

forming a gate insulating film on the second semiconductor region;

forming a gate electrode on the gate insulating film;

implanting an impurity having a second conductivity type into the secondsemiconductor region using the gate electrode as a mask to form a thirdsemiconductor region having the second conductivity type in a surface ofthe second semiconductor region;

forming an insulating film on the second semiconductor region to coverthe gate electrode;

removing a partial region of the insulating film on the gate electrode;

patterning the gate electrode using the insulating film as a mask toform an opening portion reaching the gate insulating film in a partialregion of the gate electrode;

implanting the impurity having the second conductivity type from theopening portion into the second semiconductor region to form the fourthsemiconductor region having the second conductivity type in the surfaceof the second semiconductor region, the fourth semiconductor regionbeing separated from the third semiconductor region;

removing the second and fourth semiconductor regions immediately underthe opening portion using the insulating film and gate electrode as amask to form a trench extending to the first semiconductor regionthrough the second and fourth semiconductor regions; and

filling the trench with a conductive member.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a sectional view of a conventional MOS transistor;

FIG. 2 is a sectional view of a MOS transistor according to the firstembodiment of the present invention;

FIGS. 3A to 3I are sectional views sequentially showing the steps inmanufacturing the MOS transistor according to the first embodiment ofthe present invention;

FIG. 4 is a sectional view of a MOS transistor according to the secondembodiment of the present invention;

FIGS. 5A to 5C are sectional views sequentially showing the steps inmanufacturing the MOS transistor according to the second embodiment ofthe present invention;

FIGS. 6A and 6B are sectional views sequentially showing the steps inmanufacturing a MOS transistor according to the first modification tothe second embodiment of the present invention;

FIG. 7 is a sectional view of a MOS transistor according to the secondmodification to the second embodiment of the present invention;

FIG. 8 is a sectional view of a MOS transistor according to the thirdembodiment of the present invention;

FIG. 9 is a sectional view showing some of the steps in manufacturingthe MOS transistor according to the third embodiment of the presentinvention;

FIG. 10 is a sectional view of a MOS transistor according to the fourthembodiment of the present invention;

FIGS. 11A to 11D are sectional views sequentially showing the steps inmanufacturing the MOS transistor according to the fourth embodiment ofthe present invention;

FIGS. 12A to 12C are sectional views sequentially showing the steps inmanufacturing a MOS transistor according to a modification to the fourthembodiment of the present invention;

FIGS. 13A and 13B are sectional views of MOS transistors according tothe first and second modifications to the first to fourth embodiments ofthe present invention;

FIGS. 14A to 14C are sectional views of MOS transistors according to thethird to fifth modifications to the first to fourth embodiments of thepresent invention;

FIGS. 15A to 15C are sectional views of MOS transistors according to thesixth to eighth modifications to the first to fourth embodiments of thepresent invention;

FIGS. 16A and 16B are sectional views of MOS transistors according tothe ninth and 10th modifications to the first to fourth embodiments ofthe present invention;

FIG. 17 is a sectional view of a MOS transistor according to the fifthembodiment of the present invention;

FIG. 18 is a sectional view of a MOS transistor according to the sixthembodiment of the present invention;

FIG. 19 is a sectional view of a MOS transistor according to the seventhembodiment of the present invention;

FIGS. 20A and 20B are sectional views of MOS transistors according tothe first and second modifications to the fifth to seventh embodimentsof the present invention;

FIGS. 21A to 21C are sectional views of MOS transistors according to thethird to fifth modifications to the fifth to seventh embodiments of thepresent invention;

FIGS. 22A to 22C are sectional views of MOS transistors according to thesixth to eighth modifications to the fifth to seventh embodiments of thepresent invention;

FIG. 23 is a sectional view of a MOS transistor according to the eighthembodiment of the present invention;

FIG. 24 is a sectional view of a MOS transistor according to amodification to the eighth embodiment of the present invention; and

FIGS. 25A and 25B are sectional views of MOS transistors according tomodifications to the first to eighth embodiments of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 2 is a sectional view of a MOS transistor according to the firstembodiment of the present invention. FIG. 2 especially shows thesectional structure of an offset-gate-type MOS transistor.

As shown in FIG. 2, a p⁻-type epitaxially grown layer 31 is formed on ap⁺-type silicon substrate 30. P-type body regions 32 are selectivelyformed in the surface region of the p⁻-type epitaxially grown layer 31.An n⁺-type source region 33 is selectively formed in the surface regionof each body region 32. An n⁺-type drain region 34 is selectively formedin the surface region of the epitaxial layer 31 between the adjacentbody regions 32. An n-type drift region 35 is formed in the surfaceregion of the epitaxially grown layer 31 to surround each n⁺-type drainregion 34 while being separated from the body regions 32. A gateinsulating film 37 is formed on each body region 32 and epitaxiallygrown layer 31 between the source region 33 and the drift region 35. Agate electrode is formed on the gate insulating film 37. Trenches 38extending from the surfaces of the source regions 33 between theadjacent gate electrodes 36 to the silicon substrate 30 are formed inthe epitaxially grown layer 31. Each trench 38 is filled with aconductive member 39. Each trench 38 has an opening width d (trench)substantially equal to a distance d (gate) between the adjacent gateelectrodes 36. In addition, the extended surface of the inner wall ofeach trench is flush with the extended surface of one side surface ofeach gate electrode 36. A dielectric interlayer 40 is formed on thesilicon substrate 30 to cover the gate electrodes 36 and conductivemembers 39. Contact holes 41 reaching the drain regions 34 are formed inthe dielectric interlayer 40. A drain electrode 42 is buried in eachcontact hole 41. A drain interconnection layer 43 electrically connectedto the drain electrodes 42 is formed on the dielectric interlayer 40. Asource electrode 44 is formed on the lower surface of the siliconsubstrate 30.

The MOS transistor according to this embodiment has the above-describedstructure. A power MOS transistor includes a number of unit cells havingthe above structure to increase the channel width, thereby enablinglarge current operation.

A method of manufacturing the MOS transistor according to thisembodiment will be described next with reference to FIGS. 3A to 3I.FIGS. 3A to 3I are sectional views sequentially showing the steps inmanufacturing the offset-gate-type MOS transistor. FIGS. 3A to 3Iespecially show a region AA1 (a region including two unit cells) in FIG.2.

First, as shown in FIG. 3A, the p⁻-type silicon layer 31 having athickness of about 3 μm is formed on the upper surface of the p⁺-typesilicon substrate 30 by epitaxial growth. The gate insulating film 37such as a silicon dioxide film having a thickness of about 30 nm isformed on the epitaxially grown layer 31. A polysilicon film 45 having athickness of about 0.4 μm is formed on the gate insulating film 37. Animpurity such as phosphorus is implanted into the polysilicon film 45by, e.g., vapor phase diffusion to reduce the resistance. Thepolysilicon film 45 is finally formed into gate electrodes. A resistpattern is formed on the polysilicon film 45 by photolithography. Thepolysilicon film 45 is patterned using the resist pattern as a mask. Inaddition, phosphorus-implanted layers 35-1 are formed by ion-implantingan n-type impurity such as phosphorus into the epitaxially grown layer31 to form the n-type drift regions 35, thereby obtaining the structureshown in FIG. 3A.

As shown in FIG. 3B, a resist 46 is applied to the polysilicon film 45and gate insulating film 37. The resist 46 is patterned such thatopening portions are formed in regions where the drain regions 34 are tobe formed. An n-type impurity such as arsenic to form the n⁺-type drainregions 34 is ion-implanted into the epitaxially grown layer 31 usingthe resist 46 as a mask, thereby forming arsenic-implanted layers 34-1.After that, the resist 46 is ashed and removed by the O₂-ashing method.

As shown in FIG. 3C, an insulating film 47 such as a silicon dioxidefilm having a thickness of about 1.0 μm is formed on the polysiliconfilm 45 and gate insulating film 37 by CVD (Chemical Vapor Deposition).An insulating film at a portion corresponding to a region where thetrench 38 is to be formed between adjacent gate electrodes is removed byphotolithography and etching, thereby forming an opening portion 48. Theopening width of the opening portion 48 is, e.g., about 0.5 μm.

Next, the polysilicon film 45 is etched by RIE (Reactive Ion Etching)using, e.g., Cl₂ gas. At this time, the insulating film 47 functions asa mask. As a result, the gate electrodes 36 are formed, as shown in FIG.3D.

As shown in FIG. 3E, a p-type impurity such as boron to form the p-typebody region 32 and an n-type impurity such as arsenic to form then⁺-type source region 33 are ion-implanted into the epitaxially grownlayer 31 using the insulating film 47 as a mask, thereby forming aboron-implanted layer 32-1 and arsenic-implanted layer 33-1.

Next, annealing is executed at a temperature of about 1,000° C. todiffuse the impurities in the implanted layers 32-1 to 35-1.Consequently, the p-type body region 32, n⁺-type source region 33,n⁺-type drain regions 34, and n-type drift regions 35 are completed, asshown in FIG. 3F.

Subsequently, as shown in FIG. 3G, using the gate electrodes 36 andinsulating film 47 as an etching mask, the gate insulating film 37between the adjacent gate electrodes 36 is removed by RIE using a gasmixture of CHF₃ and SF₆ or a gas mixture of CF₄ and H₂. if both the gateinsulating film 37 and insulating film 47 are formed from silicondioxide films, both films are etched. Hence, the insulating film 47 musthave an extra thickness in advance. In addition, the epitaxially grownlayer 31 is etched by RIE using HBr or SF₆ gas to form the trench 38extending from the surface of the source region 33 to the p⁺-typesilicon substrate 30 through the source region 33 and body region 32. Atthis time as well, the gate electrodes 36 and insulating film 47function as an etching mask. Hence, the opening width of the trench 38substantially equals the distance between the adjacent gate electrodes36 and is, e.g., about 0.5 μm.

The conductive member 39 made of, e.g., tungsten (W) is formed on theinsulating film 47 and in the trench 38. The conductive member 39 on theinsulating film 47 and in the opening portion 48 is etched back by CDE(Chemical Dry Etching), thereby obtaining a structure in which theconductive member 39 remains only in the trench 38, as shown in FIG. 3H.To prevent any electrical short circuit between the conductive member 39and the gate electrodes 36, etch back must be executed such that theupper end portion of the conductive member 39 is at a lower level thanthe bottom surface of each gate electrode 36. If tungsten is to beburied into the trench 38, a thin barrier metal film of, e.g., TiN ispreferably formed on the inner wall of the trench 38 before depositionof tungsten. This improves the contact between tungsten and silicon.

Next, as shown in FIG. 3I, an insulating film 49 such as a silicondioxide film is formed in the opening portion 48 between the adjacentgate electrodes 36 and on the insulating film 47. The insulating films47 and 49 form the dielectric interlayer 40.

After that, the contact hole 41 reaching the drain region 34 is formedin the dielectric interlayer 40 by photolithography and etching. Thedrain electrode 42 connected to the drain region 34 by ohmic contact isformed in the contact hole 41. The source electrode 44 is formed on thelower surface of the silicon substrate 30. The drain interconnectionlayer 43 is formed on the dielectric interlayer 40. Thus, the MOStransistor shown in FIG. 2 is completed.

According to the MOS transistor having the above structure, thefollowing effects are obtained.

(1) The MOS transistor size can be reduced.

In the MOS transistor having the structure shown in FIG. 2, the sourceregion 33 and silicon substrate 30 are electrically connected by theconductive member 39 buried in the trench 38 (the structure includingthe trench 38 and conductive member 39 will be referred to as a trenchelectrode hereinafter). In addition, the opening width d (trench) of thetrench 38 substantially equals the distance d (gate) between theadjacent gate electrodes 36. In other words, each gate electrode 36 islaid out to be adjacent to the trench 38 such that the extended surfaceof one side surface becomes flush with the extended surface of the innerwall of the trench 38. The other side surface of each gate electrode 36extends onto the surface of the corresponding n-type drift region 35. Inthe prior art, when a reach through layer is used to connect a sourceregion and silicon substrate, the reach through layer becomes as wide asabout 10 μm. This is because the reach through layer is formed byimpurity diffusion. However, according to this embodiment, the sourceregion and silicon substrate are electrically connected by the trenchelectrode. The size of the trench electrode is determined by etchingsuch as RIE in forming the trench 38. Hence, the trench width can beformed as small as about 0.5 to 1 μm or up to the exposure limitwavelength. As a result, the MOS transistor size can be greatly reduced.For example, the area of a conventional power MOS transistor including aplurality of MOS transistors is about 6 mm². With the structureaccording to this embodiment, the area decreases to about 4 mm². Thatis, the occupied area can be decreased by about 30%.

(2) The ruggedness to avalanche current of the MOS transistor can beimproved.

The trench electrode is arranged in the vicinity of each gate electrode36. A side surface of the trench electrode matches one side surface ofthe corresponding gate electrode 36. For this reason, the area of thesource region 33 also becomes smaller than the prior art. Since the areaof the body region 32 immediately under the source region 33 alsobecomes small, the resistance value of the body region 32 with respectto holes flowing at that portion in the horizontal direction becomesconsiderably small. Hence, the ruggedness to avalanche current of theMOS transistor can be improved.

(3) The MOS transistor assembly step can be simplified.

As described above, the source electrode 44 is formed on the lowersurface of the silicon substrate 30. Hence, the MOS transistor isassembled such that the source electrode 44 comes into contact with theupper surface of an assembly substrate. That is, instead of wire-bondingthe source electrode 44 to an interconnection surface of the assemblysubstrate which supplies a source potential, the surface of the sourceelectrode 44 is directly bonded to the interconnection surface. Hence,the problem of a parasitic inductance generated by bonding wires can besolved. In addition, the assembly step can be simplified. Furthermore,since the source electrode 44 is formed on the lower surface of thesilicon substrate 30, the resistance of the MOS transistor can bedecreased.

(4) Manufacturing process is facilitated.

According to the above manufacturing method, the trench 38 is formedusing the insulating film 47 and gate electrodes 36 as a mask. Hence,the inner wall of the trench 38 self-aligns with one side surface ofeach gate electrode 36. Hence, the structure can easily be implemented.

In the above embodiment, tungsten is used as the material of theconductive member 39. The tungsten can be formed by, e.g., blanket-CVDor selective growth. Tungsten is most preferably used to fill the trench38 because tungsten has an excellent filling characteristic for thetrench and is hardly be affected by high-temperature treatment such asgettering in the post-process. The filling material of the trench is notlimited to tungsten. Any other material that has a resistance lower thanthe p⁻-type epitaxially grown layer 31 can be used. Especially, arefractory metal is preferably used. Not a metal but a polycrystallinesemiconductor heavily doped with a p-type impurity may be used. When apolycrystalline semiconductor is used, a metal electrode must bepartially formed at the interface between the semiconductor member andthe n⁺-type source region 33 to short-circuit the semiconductor memberto the n⁺-type source region 33.

A MOS transistor according to the second embodiment of the presentinvention will be described next with reference to FIG. 4. FIG. 4 is asectional view of an offset-gate-type MOS transistor.

As shown in FIG. 4, a p⁻-type epitaxially grown layer 31 is formed on ap⁺-type silicon substrate 30. P-type body regions 32 are selectivelyformed in the surface region of the p⁻-type epitaxially grown layer 31.An n⁺-type source region 33 is selectively formed in the surface regionof each body region 32. An n⁺-type drain region 34 is selectively formedin the surface region of the epitaxial layer 31 between the adjacentbody regions 32. An n-type drift region 35 is formed in the surfaceregion of the epitaxially grown layer 31 to surround each n⁺-type drainregion 34 while being separated from the body regions. A gate insulatingfilm 37 is formed on each body region 32 and epitaxially grown layer 31between the source region 33 and the drift region 35. A gate electrode36 is formed on the gate insulating film 37.

An insulating film 50 is formed at least on the source-region-sidesidewall of each gate electrode 36. Trenches 38 extending from thesurfaces of the source regions 33 between the adjacent gate electrodes36 to the silicon substrate 30 are formed in the epitaxially grown layer31. Each trench 38 is filled with a conductive member 39. Each trench 38has an opening width d (trench) substantially equal to a distance d(wall) between the insulating films 50 formed on the sidewalls of theadjacent gate electrodes 36. In addition, the extended surface of theinner wall of each trench is flush with the extended surface of one sidesurface of each insulating film 50. A dielectric interlayer 40 is formedon the silicon substrate 30 to cover the gate electrodes 36 andconductive members 39. Contact holes 41 reaching the drain regions 34are formed in the dielectric interlayer 40. A drain electrode 42 isburied in each contact hole 41. A drain interconnection layer 43electrically connected to the drain electrodes 42 is formed on thedielectric interlayer 40. A source electrode 44 is formed on the lowersurface of the silicon substrate 30.

A method of manufacturing the MOS transistor according to thisembodiment will be described next with reference to FIGS. 5A to 5C.FIGS. 5A to 5C are sectional views sequentially showing the steps inmanufacturing the offset-gate-type MOS transistor. FIGS. 5A to 5Cespecially show a region AA1 (a region including two unit cells) in FIG.4.

First, the structure shown in FIG. 3E is formed by in accordance withthe procedure described in the first embodiment. The distance d (gate)between adjacent gate electrodes is, e.g., 0.7 μm.

Next, annealing is executed at a temperature of about 1,000° C. todiffuse the impurities in implanted layers 32-1 to 35-1. Consequently,the p-type body region 32, n⁺-type source region 33, n⁺-type drainregions 34, and n-type drift regions 35 are completed. Simultaneously,the sidewall portion of each gate electrode 36, which is exposed into anopening portion 48, is oxidized. With this oxidation, the insulatingfilms 50 (oxide films) are formed on the sidewalls of the gateelectrodes 36, thereby obtaining the structure shown in FIG. 5A. Thethickness of the insulating film 50 is, e.g., 0.1 μm.

Next, as shown in FIG. 5B, using gate insulating films 47 and 50 as anetching mask, the gate insulating film 37 between the adjacent gateelectrodes 36 is removed by RIE using a gas mixture of CHF₃ and SF₆ or agas mixture of CF₄ and H₂. In addition, the epitaxially grown layer 31is etched by RIE using HBr or SF₆ gas to form the trench 38 extendingfrom the surface of the source region 33 to the p⁺-type siliconsubstrate 30. At this time as well, the insulating films 47 and 50function as an etching mask. Hence, the opening width of the trench 38substantially equals the distance between the adjacent insulating films50 and is, e.g., about 0.5 μm.

As shown in FIG. 5C, the conductive member 39 made of, e.g., tungsten(W) is buried into the trench 38.

After that, the structure shown in FIG. 4 is completed by the stepdescribed in the first embodiment with reference to FIG. 3I.

According to the MOS transistor having the above structure, thefollowing effect (5) is obtained in addition to the effects (1) to (4)described in the first embodiment.

(5) The manufacturing process is further facilitated.

In the MOS transistor according to the structure and manufacturingmethod of this embodiment, the insulating film 50 is formed on thesidewall of each gate electrode 36. For this reason, the gate electrode36 is surrounded by the insulating film. Hence, even if the uppersurface of the conductive member 39 buried in the trench 38 is at ahigher level than the bottom surface of each gate electrode 36, i.e.,the etch back step for the conductive member 39 has been insufficient,any electrical short circuit between the source region 33 and the gateelectrode 36 can be prevented. Since the burying height of theconductive member 39 is not particularly limited, formation of thetrench electrode is very easy.

The insulating film 50 on the gate electrode sidewall need not always beformed by oxidation. Another method of forming the insulating film 50will be described with reference to FIGS. 6A and 6B as the firstmodification to this embodiment. FIGS. 6A and 6B are sectional views ofthe steps in manufacturing a MOS transistor.

First, the structure shown in FIG. 3F is formed by the manufacturingsteps of the first embodiment. After that, as shown in FIG. 6A, theinsulating film 50 having a thickness of, e.g., 0.1 μm is formed in theopening portion 48 and on the insulating film 47 by CVD.

The insulating film 50 on the insulating film 47 and gate insulatingfilm 37 is etched back to leave the insulating film 50 only on thesidewall of each gate electrode 36, thereby obtaining the structureshown in FIG. 6B.

Even with this method, the insulating film 50 can be formed on the gateelectrode sidewall. In this case, in forming the insulating film 50 inFIG. 6A, the opening portion 48 must be prevented from being completelyfilled with the insulating film 50.

An example in which the insulating film 50 is formed by still anothermethod will be described with reference to FIG. 7 as the secondmodification to this embodiment. FIG. 7 is a sectional view of a MOStransistor.

As shown in FIG. 7, the structure according to this modification uses,as the insulating film 50, a sidewall that is widely used in a CMOS(Complementary MOS) structure or the like. In this case as well, thesame effects as described above can be obtained.

A MOS transistor according to the third embodiment of the presentinvention will be described next with reference to FIG. 8. FIG. 8 is asectional view of an offset-gate-type MOS transistor. For thesimplicity, a description of the same parts as in the structure shown inFIG. 2 described in the first embodiment will be omitted.

As shown in FIG. 8, the MOS transistor according to this embodimentfurther comprises p⁺-type semiconductor layers 51 in the structure shownin FIG. 2. Each p⁺-type semiconductor layer 51 is formed in anepitaxially grown layer 31 along the sidewall of a trench 38 while beingin contact with a source region 33 and silicon substrate 30. A bodyregion 32 is electrically connected to a conductive member 39 throughthe semiconductor layer 51 with a low resistance.

A method of manufacturing the MOS transistor having the above structurewill be described next with reference to FIG. 9. FIG. 9 is a sectionalview showing some of the steps in manufacturing the MOS transistor.

The structure shown in FIG. 3G described in the first embodiment isformed. After that, as shown in FIG. 9, a p-type impurity such as boronis ion-implanted into the epitaxially grown layer 31 and body region 32on the sidewall of the trench 38 at a high concentration by oblique ionimplantation. When the impurity is activated by annealing, the p⁺-typesemiconductor layer 51 can be formed.

After that, the structure shown in FIG. 8 is completed by themanufacturing steps described in the first embodiment with reference toFIGS. 3H and 3I.

Even according to the MOS transistor having the above structure, theeffects (1) to (4) described in the first embodiment are obtained. Thep⁺-type semiconductor layer 51 is formed in a region immediately underthe source region 33. The body region 32 is electrically connected tothe conductive member 39 through the p⁺-type semiconductor layer 51. Inother words, the p⁺-type semiconductor layer 51 serves as part of thebody region. The resistance of the body region becomes low, and thecontact resistance between the body region and the conductive member isdecreased. As a result, the ruggedness to avalanche current of the MOStransistor can be further improved.

A MOS transistor according to the fourth embodiment of the presentinvention will be described next with reference to FIG. 10. FIG. 10 is asectional view of an offset-gate-type MOS transistor. For thesimplicity, a description of the same parts as in the structure shown inFIG. 2 described in the first embodiment will be omitted.

As shown in FIG. 10, the MOS transistor according to this embodimentfurther comprises p⁺-type semiconductor layers 52 in the structure shownin FIG. 2. Each p⁺-type semiconductor layer 52 is formed in anepitaxially grown layer 31 along the sidewall of a trench 38. The uppersurface of the semiconductor layer 52 is in contact with the bottomportion of a body region 32. The body region 32 is electricallyconnected to a conductive member 39 through the semiconductor layer 52with a low resistance.

A method of manufacturing the MOS transistor having the above structurewill be described next with reference to FIGS. 11A to 1D. FIGS. 11A to11D are sectional views sequentially showing the steps in manufacturingthe MOS transistor.

First, the structure shown in FIG. 3D is obtained by the steps describedin the first embodiment.

Next, as shown in FIG. 11A, a p-type impurity such as boron to form thep-type body region 32, an n-type impurity such as arsenic to form ann⁺-type source region 33, and a p-type impurity such as boron to formthe p⁺-type semiconductor layer 52 are ion-implanted into theepitaxially grown layer 31 using an insulating film 47 as a mask,thereby forming a boron-implanted layer 32-1, arsenic-implanted layer33-1, and boron-implanted layer 52-1. The boron-implanted layer 52-1 isformed to be deeper than the boron-implanted layer 32-1 andarsenic-implanted layer 33-1 by adjusting the acceleration voltage inion implantation. For example, the boron-implanted layer 32-1 andarsenic-implanted layer 33-1 are formed by ion implantation atacceleration voltages of about 60 kV and 50 kV, respectively. Theboron-implanted layer 52-1 is formed by ion implantation at anacceleration voltage of about 500 kV.

As shown in FIG. 11B, annealing is executed at a temperature of about1,000° C. to diffuse the impurities in the implanted layers 32-1 to 35-1and 52-1, as in the first embodiment. Consequently, the p-type bodyregion 32, n⁺-type source region 33, n⁺-type drain regions 34, n-typedrift regions 35, and p⁺-type semiconductor layer 52 are completed.

Next, as shown in FIG. 11C, using gate electrodes 36 and insulating film47 as an etching mask, a gate insulating film 37 between the adjacentgate electrodes 36 is removed by RIE using a gas mixture of CHF₃ and SF₆or a gas mixture of CF₄ and H₂. When both the gate insulating film 37and insulating film 47 are formed from silicon dioxide films, both filmsare etched. Hence, the insulating film 47 must have an extra thicknessin advance. In addition, the epitaxially grown layer 31 is etched by RIEusing HBr or SF₆ gas to form the trench 38 extending from the surface ofthe source region 33 to a p⁺-type silicon substrate 30 through the bodyregion 32 and p⁺-type semiconductor layer 52. At this time as well, thegate electrodes 36 and insulating film 47 function as an etching mask.Hence, the opening width of the trench 38 substantially equals thedistance between the adjacent gate electrodes 36.

As shown in FIG. 11D, the conductive member 39 made of, e.g., tungstenis formed on the insulating film 47 and in the trench 38. The conductivemember 39 on the insulating film 47 and in an opening portion 48 isetched back by CDE, thereby obtaining a trench electrode in which theconductive member 39 remains only in the trench 38, as shown in FIG.11D.

After that, an insulating film 49 such as a silicon dioxide film isformed in the opening portion 48 between the adjacent gate electrodes 36and on the insulating film 47. The insulating films 47 and 49 form adielectric interlayer 40. A contact hole 41 reaching the drain region 34is formed in the dielectric interlayer 40. A drain electrode 42 isformed in the contact hole 41. A source electrode 44 is formed on thelower surface of the silicon substrate 30. A drain interconnection layer43 is formed on the dielectric interlayer 40. Thus, the MOS transistorshown in FIG. 10 is completed.

According to the MOS transistor having the above structure, the effects(1) to (4) described in the first embodiment can be obtained. Inaddition, since the p⁺-type semiconductor layer 52 is formed, the effectdescribed in the third embodiment can be obtained. That is, since theresistance to holes immediately under the n⁺-type source region 33 canbe reduced, and the contact resistance between the conductive member 39and the body region 32 can be decreased, the ruggedness to avalanchecurrent of the MOS transistor can be further improved.

The method of forming the p⁺-type semiconductor layer 52 is not limitedto the above method. Another method of forming the p⁺-type semiconductorlayer 52 will be described with reference to FIGS. 12A to 12C as amodification to this embodiment. FIGS. 12A to 12C are sectional viewssequentially showing the steps in manufacturing a MOS transistor.

First, the structure shown in FIG. 3F is formed by the steps describedin the first embodiment. Next, as shown in FIG. 12A, using the gateelectrodes 36 and insulating film 47 as an etching mask, the gateinsulating film 37 between the adjacent gate electrodes 36 is removed byRIE using a gas mixture of CHF₃ and SF₆ or a gas mixture of CF₄ and H₂.In addition, the epitaxially grown layer 31 is etched by RIE using HBror SF₆ gas to form the trench 38 extending from the surface of thesource region 33 to the p⁻-type epitaxially grown layer 31 through thesource region 33 and body region 32. At this time as well, the gateelectrodes 36 and insulating film 47 function as an etching mask.

As shown in FIG. 12B, a p-type impurity such as boron to form thep⁺-type semiconductor layer 52 is ion-implanted from the bottom surfaceof the trench 38 into the epitaxially grown layer 31 using theinsulating film 47 as a mask. Annealing is executed at a temperature ofabout 1,000° C. to diffuse the impurity, thereby forming the p⁺-typesemiconductor layer 52. The p⁺-type semiconductor layer 52 must beformed in contact with the bottom portion of the body region 32.

As shown in FIG. 12C, the epitaxially grown layer 31 exposed to thebottom surface of the trench 38 is etched by RIE using HBr or SF₆ gasuntil etching reaches the silicon substrate 30 through the body region32 and p⁺-type semiconductor layer 52. At this time as well, the gateelectrodes 36 and insulating film 47 function as an etching mask.

After that, the conductive member 39 made of, e.g., tungsten is formedon the insulating film 47 and in the trench 38. The conductive member 39on the insulating film 47 and in the opening portion 48 is etched backby CDE, thereby obtaining a trench electrode in which the conductivemember 39 remains only in the trench 38. Additionally, the insulatingfilm 49 such as a silicon dioxide film is formed in the opening portion48 between the adjacent gate electrodes 36 and on the insulating film47. The insulating films 47 and 49 form the dielectric interlayer 40.The contact hole 41 reaching the drain region 34 is formed in thedielectric interlayer 40. The drain electrode 42 is formed in thecontact hole 41. The source electrode 44 is formed on the lower surfaceof the silicon substrate 30. The drain interconnection layer 43 isformed on the dielectric interlayer 40. Thus, the MOS transistor shownin FIG. 10 is completed.

A MOS transistor according to the first modification to the first tofourth embodiments will be described next with reference to FIG. 13A.FIG. 13A is a sectional view of an offset-gate-type MOS transistor.

In this modification, the second and third embodiments are combined. Asshown in FIG. 13A, in the structure wherein the insulating film 50 isformed on the sidewall portion of each gate electrode 36, the p⁺-typesemiconductor layer 51 may be formed between the source region 33 andthe silicon substrate 30. According to this modification, all theeffects of the second and third embodiments can be obtained.

FIG. 13B is a sectional view of an offset-gate-type MOS transistoraccording to the second modification to the first to fourth embodiments.

In this modification, the second and fourth embodiments are combined. Asshown in FIG. 13B, in the structure wherein the insulating film 50 isformed on the sidewall portion of each gate electrode 36, the p⁺-typesemiconductor layer 52 may be formed in the region immediately under thebody region 32. According to this modification, all the effects of thesecond and fourth embodiments can be obtained.

FIGS. 14A to 14C are sectional views of offset-gate-type MOS transistorsaccording to the third to fifth modifications to the first to fourthembodiments.

The MOS transistors according to these modifications areoffset-gate-type MOS transistors as in the first to fourth embodiments.However, the p-type body region 32 is formed in contact with the n-typedrift region 35. This structure can be implemented by forming the bodyregion 32 before the gate electrodes 36. In FIGS. 14A to 14C, the first,third, and fourth embodiments are applied to a MOS transistor having theabove structure.

According to these modifications, in addition to the effects describedin the above embodiments, a punch through phenomenon hardly occursalthough the channel length increases.

FIGS. 15A to 15C are sectional views of offset-gate-type MOS transistorsaccording to the sixth to eighth modifications to the first to fourthembodiments.

The MOS transistors according to these modifications areoffset-gate-type MOS transistors as in the first to fourth embodiments.However, the drift region 35 is formed to be shallower than the n⁺-typedrain region 34. In FIGS. 15A to 15C, the first, third, and fourthembodiments are applied to a MOS transistor having the above structure.

The second embodiment may be applied to the above modifications. Thatis, in the structures shown in FIGS. 14A to 14C and 15A to 15C, theinsulating film 50 may be formed on the sidewall portion of each gateelectrode 36.

FIG. 16A is a sectional view of an offset-gate-type MOS transistoraccording to the ninth modification to the first to fourth embodiments.

In this modification, the opening width d (trench) of the trench 38becomes smaller as the depth increases. When a trench is formed by RIE,normally, the opening width d (trench) becomes smaller as the depthincreases. In the first to fourth embodiments, the trench opening widthmay change in accordance with the depth. The trench opening width ratherpreferably decreases as the depth increases because the fillingproperties in the trench improve.

FIG. 16B is a sectional view of an offset-gate-type MOS transistoraccording to the 10th modification to the first to fourth embodiments.In this modification, the opening width d (trench) of the trench 38 islarger at its opening end portion than the width d (gate) between theadjacent gate electrodes 36.

As shown in FIG. 16B, even if the trench electrode partially enters thelower portions of the gate electrodes 36, the same effects as in thefirst to fourth embodiments can be obtained.

The trench electrode serves as a reach through layer which electricallyconnects the source region 33 and silicon substrate 30 in theconventional structure. Hence, the effects are obtained by forming thetrench electrode narrower than the reach through layer that is formed byimpurity diffusion. When the distance between the gate electrodes isdecreased, the transistor size can be reduced. For this reason, if theopening width can be decreased at the opening end of the trench, asufficient effect can be obtained. A more preferable form is obtainedwhen d (trench)=d (gate), as described in the first to fourthembodiments. Hence, d (trench) may be slightly different from d (gate),e.g., d (trench)<d (gate). It suffices that d (trench) almost equals d(gate).

Hence, the description in this specification, “the trench 38 has theopening width d (trench) substantially equal to the distance d (gate)between the adjacent gate electrodes 36” or “each gate electrode 36 islaid out to be adjacent to the trench 38 such that the extended surfaceof one side surface becomes flush with the extended surface of the innerwall of the trench 38” is sufficiently satisfied when “at least theopening end of the trench 38 substantially equals the distance betweenthe gate electrodes” or “the trench 38 is formed using the gateelectrodes 36 as a mask”.

A MOS transistor according to the fifth embodiment of the presentinvention will be described next with reference to FIG. 17. FIG. 17 is asectional view of an offset-gate-type MOS transistor.

As shown in FIG. 17, p-type body regions 32 are selectively formed inthe surface region of a p-type epitaxially grown layer 31 on a p⁺-typesilicon substrate 30. An n⁺-type source region 33 is formed in thesurface region of each body region 32. A p⁺-type contact layer 53 isformed in contact with each source region 33. An n⁺-type drain region isformed in the surface region of the p-type epitaxially grown layer 31between the adjacent body regions 32. N-type drift regions 35 are formedfrom the drain region 34 along the body regions 32 while being separatedfrom the body regions 32. A gate insulating film 37 is formed on eachsource region 32 between the source region 33 and the drift region 33. Agate electrode 36 is formed on the gate insulating film 37. Aninsulating film 54 surrounds the upper and side surfaces of each gateelectrode 36. A source electrode 55 is formed on the source region 33and contact layer 53 to electrically short-circuit them. The sourceelectrode 55 surrounds the upper and side surfaces of the insulatingfilm 54 that covers the gate electrode 36. The source electrode 55 isinsulated from the drain region 34 and drift region 35 by an insulatingfilm 56. That is, the upper and side surfaces of the gate electrode 36are surrounded by the source electrode 55. The insulating film 56 may bepart of the insulating film 54. A dielectric interlayer 40 is formed onthe epitaxially grown layer 31 to cover the source electrodes 55 andinsulating films 56. A contact hole 41 is formed to extend from thesurface of the dielectric interlayer 40 to the drain region 34 throughthe insulating film 56. A drain electrode (drain interconnection layer)43 fills the contact hole 41 and extends on the dielectric interlayer40. The drain interconnection layer 43 on the dielectric interlayer 40extends on the gate electrodes 36 and source electrodes 55 (the draininterconnection layer 43 overlaps the gate electrodes 36 and sourceelectrodes 55). A p⁺-type reach through layer 57 is formed in the p-typeepitaxially grown layer 31 from a portion immediately under each sourceelectrode 55 to the p⁺-type silicon substrate 30. With this structure,each source region 33 is electrically connected to the p⁺-type siliconsubstrate 30. A source electrode 44 is formed on the lower surface ofthe p⁺-type silicon substrate 30.

In the MOS transistor shown in FIG. 17, when a forward voltage isapplied between the source and the drain and to the gate, a channel isformed in the upper surface of the body region 32 between the sourceregion 33 and the drain region 34. A current that flows from the drainregion 34 to the source region 33 through the drift region 35 andchannel flows into the source electrode 44 through the source electrode55, contact layer 53, reach through layer 57, and p⁺-type siliconsubstrate 30.

With the MOS transistor having the above structure, the following effect(6) can be obtained.

(6) The feedback capacitance of the MOS transistor can be decreased.

In the structure shown in FIG. 17, the source electrode 55 is insertedbetween each gate electrode 36 and the drain interconnection layer 43which is present immediately above the gate electrode 36 and on the sideportion of the gate electrode 36. The source electrode 55 is normallyfixed at the ground potential. As a result, since the source electrode55 serves as a shield, the parasitic capacitance between the gate andthe drain of the MOS transistor can be reduced. If the parasiticcapacitance can be reduced, a power loss in high-frequency operationsuch as high-speed switching of the MOS transistor can be reduced.

Additionally, the effect (3) described in the first embodiment can beobtained. That is, wire bonding the source electrode to the assemblysubstrate in the assembly step is unnecessary. For this reason, theproblem of a parasitic inductance generated by bonding wires can besolved. In addition, the assembly step can be simplified. Furthermore,if the source region 33 and p⁺-type silicon substrate 30 are connectedby the heavily doped reach through layer 57, the resistance of the MOStransistor can be decreased.

In the MOS transistor according to this embodiment, the contact hole 41is filled with the drain electrode 43 that also serves as a draininterconnection layer. However, the drain electrode 43 may be formed byfilling the contact hole 41 with a metal plug or the like, and then, ametal interconnection layer serving as a drain interconnection layer maybe newly formed on the dielectric interlayer 40.

A MOS transistor according to the sixth embodiment of the presentinvention will be described next with reference to FIG. 18. FIG. 18 is asectional view of an offset-gate-type MOS transistor. The structure in ap-type epitaxially grown layer 31 is the same as in the fifthembodiment, and a description thereof will be omitted. Only thestructure on the p-type epitaxially grown layer 31 will be described.

A gate insulating film 37 is formed on each body region 32 between asource region 33 and a drift region 35. A gate electrode 36 is formed onthe gate insulating film 37. The gate electrode 36 is surrounded by agate insulating film 54. A source electrode 55 is formed on the sourceregion 33 and contact layer 53 to electrically short-circuit them. Thesource electrode 55 extends to a portion immediately above the gateelectrode 36. A part of the insulating film 54 is interposed between thegate electrode 36 and source electrode 55. A drain electrode 58 isformed on a drain region 34. A dielectric interlayer 40 is formed on theepitaxially grown layer 31 to cover the source electrodes 55, drainelectrode 58, and insulating films 54. A contact hole 41 is formed toextend from the surface of the dielectric interlayer 40 to the drainelectrode 58. A drain interconnection layer 43 fills the contact hole 41and extends on the dielectric interlayer 40.

With the MOS transistor having the above structure, the effect (6)described in the fifth embodiment can be obtained. That is, in the MOStransistor according to this embodiment, the source electrode 55 isinserted between each gate electrode 36 and the drain interconnectionlayer 43 which is present immediately above the gate electrode 36. Thesource electrode 55 is normally fixed at the ground potential. Since thesource electrode 55 serves as a shield, the parasitic capacitancebetween the gate and the drain of the MOS transistor can be reduced.

The drain electrode 58 can be formed by self-alignment using theinsulating film 54. This structure is suitable for miniaturizedsemiconductor device. The drain electrode 58 and drain electrode 43 maybe integrated, as shown in FIG. 17.

A MOS transistor according to the seventh embodiment of the presentinvention will be described next with reference to FIG. 19. FIG. 19 is asectional view of an offset-gate-type MOS transistor. The structure in ap-type epitaxially grown layer 31 is the same as in the fifthembodiment, and a description thereof will be omitted. Only thestructure on the p-type epitaxially grown layer 31 will be described.

A gate insulating film 37 is formed on each body region 32 between asource region 33 and a drift region 35. A gate electrode 36 is formed onthe gate insulating film 37. The gate electrode 36 is surrounded by agate insulating film 54. An insulating film 54-1 having the same heightas that of the insulating film 54 is formed between the adjacent gateelectrodes 36 while being separated from the insulating films 54 by apredetermined distance. An insulating film 54-2 thinner than theinsulating films 54 and 54-1 is also formed on each drift region 35between the adjacent insulating films 54 and 54-1. A polysilicon film 59is formed on each insulating film 54-2. A source electrode 55 is formedon the source region 33 and contact layer 53 to electricallyshort-circuit them. The source electrode 55 extends from the uppersurface of the insulating film 54 that covers the gate electrode 36 ontothe polysilicon film 59. The source electrode 55 is also electricallyconnected to the polysilicon film 59. A dielectric interlayer 40 isformed on the epitaxially grown layer 31 to cover the source electrodes55 and insulating film 54-1. A contact hole 41 is formed to extend fromthe surface of the dielectric interlayer 40 to a drain region 34 throughthe insulating film 54-1. A drain electrode (drain interconnectionlayer) 43 fills the contact hole 41 and extends on the dielectricinterlayer 40.

With the MOS transistor having the above structure, the effect (6)described in the fifth embodiment can be obtained. That is, in the MOStransistor according to this embodiment, the source electrode 55 isinserted between each gate electrode 36 and the drain interconnectionlayer 43 which is present immediately above the gate electrode 36. Thesource electrode 55 is normally fixed at the ground potential. Inaddition, the polysilicon film 59 connected to the source electrode 55and set at the same potential at that of the source electrode 55 isformed on the side surface of each gate electrode 36. Since the sourceelectrode 55 and polysilicon film 59 serve as a shield, the parasiticcapacitance between the gate and the drain of the MOS transistor can bereduced.

The seventh embodiment can be relatively easily practiced as compared tothe fifth embodiment. This is because the material of the gateelectrodes 36 can also be used to form the polysilicon films 59. Thatis, the gate electrodes are formed by forming a polysilicon film on thegate insulating film 37 and patterning the polysilicon film into a gateelectrode pattern. When the polysilicon film is patterned, thepolysilicon film is left at a portion adjacent to each gate electrode.As a result, the polysilicon films 59 can be formed.

The drain electrode 58 may be formed by filling the contact hole 41 witha metal plug or the like, and then, the drain interconnection layer 43may be newly formed on the dielectric interlayer 40, as shown in FIG. 18described in the sixth embodiment. The polysilicon film 59 may be ametal interconnection layer formed in a step different from that for thegate electrodes. The polysilicon film 59 can be formed from any otherconductive film that is set at the same potential as that of the sourceelectrode.

The source electrode 55 need not always be extracted onto the portionimmediately above the gate electrode 36, as in the fifth to seventhembodiments. Such structures will be described with reference to FIGS.20A and 20B as the first and second modifications to the fifth toseventh embodiments.

FIG. 20A is a sectional view of a MOS transistor according to the firstmodification to the fifth to seventh embodiments of the presentinvention. As shown in FIG. 20A, in a MOS transistor according to thismodification, a conductive film 60-1 is formed in place of the sourceelectrode 55 immediately above each gate electrode 36. The samepotential as that of the source electrode 55 is applied to theconductive film 60-1.

FIG. 20B is a sectional view of a MOS transistor according to the secondmodification to the fifth to seventh embodiments of the presentinvention. As shown in FIG. 20B, in a MOS transistor according to thismodification, in the structure shown in FIG. 20A described in the firstmodification, a conductive film 60-2 is formed between the drainelectrode 43 and the sidewall of each gate electrode 36. The samepotential as that of the source electrode 55 is applied to theconductive film 60-2.

When a region at the same potential as that of the source electrode 55is formed near the gate electrode 36, the same effect as in the fifth toseventh embodiments can be obtained. The conductive films 60-1 and 60-2need not always be at the same potential as that of the source electrode55. For example, even when the conductive films are set at apredetermined potential between the source potential and the drainpotential, the same effect as described above can be obtained.

FIGS. 21A to 21C are sectional views of offset-gate-type MOS transistorsaccording to the third to fifth modifications to the fifth to seventhembodiments.

The MOS transistors according to these modifications areoffset-gate-type MOS transistors as in the fifth to seventh embodiments.However, the n-type drift region 35 is formed to surround the n⁺-typedrain region 34. In FIGS. 21A to 21C, the fifth to seventh embodimentsare applied to a MOS transistor having the above structure.

Even in these modifications, the same effect as in the above embodimentscan be obtained.

FIGS. 22A to 22C are sectional views of offset-gate-type MOS transistorsaccording to the sixth to eighth modifications to the fifth to seventhembodiments.

The MOS transistors according to these modifications areoffset-gate-type MOS transistors as in the fifth to seventh embodiments.However, the n-type drift region 35 is formed to surround the n⁺-typedrain region 34. In addition, the p-type body region 32 is separatedfrom the drift region.

Even in these modifications, the same effect as in the above embodimentscan be obtained.

A MOS transistor according to the eighth embodiment of the presentinvention will be described next with reference to FIG. 23. FIG. 23 is asectional view of an offset-gate-type MOS transistor.

In this embodiment, the first and fifth embodiments are combined. Thatis, in the structure shown in FIG. 2, a conductive film 61 is formed ona dielectric interlayer 40 immediately above gate electrodes 36. Theconductive film 61 is set at the same potential as that of a sourceelectrode 44. A dielectric interlayer 62 is formed on the dielectricinterlayer 40 to cover the conductive film 61. A drain electrode 43 isformed on the dielectric interlayer 62. As a result, as shown in FIG.23, the conductive film 61 at the same potential as that of the sourceelectrode 44 is present between the gate electrodes 36 and the drainelectrode 43.

According to the MOS transistor of this embodiment, both the effects (1)to (4) described in the first embodiment and the effect (6) described inthe fifth embodiment can be obtained.

FIG. 24 is a sectional view of an offset-gate-type MOS transistoraccording to a modification to the eighth embodiment. In thismodification, the second modification (FIG. 7) to the second embodimentand the fifth embodiment are combined.

As shown in FIG. 24, in the structure shown in FIG. 7, the conductivemember 39 reaches the surface of the dielectric interlayer 40. Theconductive film 61 is formed on the dielectric interlayer 40 immediatelyabove the gate electrodes 36. The conductive film 61 is in contact withthe conductive member 39. The dielectric interlayer 62 is formed on thedielectric interlayer 40 to cover the conductive film 61. The draininterconnection layer 43 is formed on the dielectric interlayer 62. Withthis structure, the trench electrode and conductive film 61 can easilybe set at an equipotential.

The combinations of the first to fourth embodiments and the fifth toseventh embodiments are not limited to the structures shown in FIGS. 23and 24. The embodiments can be freely combined.

As described above, in the MOS transistors according to the first tofourth embodiments, the source region 33 and silicon substrate 30 areelectrically connected using the trench electrode. Each gate electrode36 is laid out to be adjacent to the trench 38 such that the extendedsurface of one side surface becomes flush with the extended surface ofthe sidewall of the trench 38. Hence, the size of the MOS transistor canbe reduced. In addition, since the area of the source region 33 can bereduced, the resistance value of the body region 32 becomes considerablysmall. Hence, the ruggedness to avalanche current of the MOS transistorcan be improved.

In the first to fourth embodiments, the drift region and drain regionare formed before the trench formation step. However, these regions maybe formed after the trench formation step. The sequence of these stepsis not particularly limited. The drift region and drain region areformed by re-diffusing the impurity-implanted layers in the annealingstep to form the body region and source region. However, the driftregion and drain region may be formed in another annealing step.

In the MOS transistors according to the fifth to seventh embodiments ofthe present invention, each gate electrode 36 is surrounded by thesource electrode 55 or by the source electrode 55 and conductive films59, 60-1, and 60-2. The same potential as that of the source electrode55 or a predetermined potential between the source potential and thedrain potential is applied to the conductive films 59, 60-1, and 60-2.For this reason, the parasitic capacitance between the gate electrode ofthe MOS transistor and the drain electrode and drain interconnectionlayer can be reduced, and the feedback capacitance can be reduced.Consequently, a power loss in high-frequency operation such ashigh-speed switching of the semiconductor device can be reduced.

In the fifth to seventh embodiments, an example has been described, inwhich a current is caused to flow to the drain region 34, drift region35, source region 33, source electrode 55, contact layer 53, reachthrough layer 57, and p⁺-type silicon substrate 30 whereby the sourceelectrode 44 is used as an actual source electrode, and the sourceelectrode 55 serves only as part of the current path. However, thesource electrode 55 may be used as an actual source electrode. That is,in assembling the semiconductor device, an interconnection layer on theassembly substrate for supplying the source potential may be connectedto the source electrode 55. The fifth to seventh embodiments can beapplied not only to an offset-gate-type MOS transistor but also to anysemiconductor device in which three electrodes having differentpotentials are present on a single surface, and a parasitic capacitancebetween two overlapping electrodes poses a problem.

In the MOS transistor according to the eighth embodiment of the presentinvention, the first to fourth embodiments and the fifth to seventhembodiments are combined, thereby obtaining all the effects of theseembodiments.

The first to eighth embodiments of the present invention can also beapplied to a pMOS transistor. In this case, the n- and p-types in theabove embodiments are reversed. In the above embodiments, the driftregion is formed to raise the breakdown voltage of the element. However,the drift region may be omitted if no high breakdown voltage isnecessary. In this case, the drain region 34 is formed such that its endportion is located immediately under the gate electrode 36, as shown inFIGS. 25A and 25B that are sectional views of MOS transistors.

The semiconductor device manufacturing methods of the above embodimentsare not limited to the orders described above. The order of the stepscan be exchanged as long as a degree of freedom can be ensured.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

1. A method for fabricating a semiconductor device comprising: forming asecond semiconductor region having a first conductivity type and aresistance higher than a first semiconductor region on the firstsemiconductor region having the first conductivity type; forming a gateinsulating film on the second semiconductor region; forming a gateelectrode on the gate insulating film; implanting an impurity having asecond conductivity type into the second semiconductor region using thegate electrode as a mask to form a third semiconductor region having thesecond conductivity type in a surface of the second semiconductorregion; forming an insulating film on the second semiconductor region tocover the gate electrode; removing a partial region of the insulatingfilm on the gate electrode; patterning the gate electrode using theinsulating film as a mask to form an opening portion reaching the gateinsulating film in a partial region of the gate electrode; implantingthe impurity having the second conductivity type from the openingportion into the second semiconductor region to form the fourthsemiconductor region having the second conductivity type in the surfaceof the second semiconductor region, the fourth semiconductor regionbeing separated from the third semiconductor region; removing the secondand fourth semiconductor regions immediately under the opening portionusing the insulating film and gate electrode as a mask to form a trenchextending to the first semiconductor region through the second andfourth semiconductor regions; and filling the trench with a conductivemember.
 2. The method according to claim 1, further comprising: beforeformation of the trench, forming a sidewall insulating film on asidewall of the gate electrode exposed into the opening portion; whereinthe trench is formed further using the sidewall insulating film as amask.
 3. The method according to claim 1, further comprising: after theopening portion reaching the gate insulating film is formed in thepartial region of the gate electrode, implanting an impurity having thefirst conductivity type from the opening portion into the surface of thesecond semiconductor region to form a fifth semiconductor region havingthe first conductivity type and a resistance lower than the secondsemiconductor region in the surface of the second semiconductor region;wherein the fifth semiconductor region surrounds the fourthsemiconductor region, and the trench is formed to further extend throughthe fifth semiconductor region.
 4. The method according to claim 3,wherein the formation of the fourth and fifth semiconductor regionsincludes: implanting the impurities having the first and secondconductivity types from the opening portion into the surface of thesecond semiconductor region; and diffusing the impurities implanted intothe second semiconductor region.
 5. The method according to claim 3,further comprising: after the opening portion reaching the gateinsulating film is formed in the partial region of the gate electrode,forming a sixth semiconductor region having the first conductivity typeand a resistance lower than the firth semiconductor region in the secondsemiconductor region immediately under the fifth semiconductor region,the sixth semiconductor region being in contact with the fifthsemiconductor region; wherein the trench is formed to further extendthrough the sixth semiconductor region.
 6. The method according to claim5, wherein the formation of the sixth semiconductor region includes:implanting the impurity having the first conductivity type from theopening portion into the second semiconductor region at a positiondeeper than that of the impurities to form the fourth and fifthsemiconductor regions; and diffusing the impurity implanted into thesecond semiconductor region.
 7. The method according to claim 3, whereinthe formation of the trench includes: after the fourth and fifthsemiconductor regions are formed, removing the fourth and fifthsemiconductor regions immediately under the opening portion using theinsulating film and gate electrode as a mask to form a first trenchextending to the fifth semiconductor region through the fourthsemiconductor region; implanting an impurity having the firstconductivity type near a bottom surface of the first trench to form asixth semiconductor region having a resistance lower than the fifthsemiconductor region; and removing the second and sixth semiconductorregions at a bottom portion of the first trench using the insulatingfilm and gate electrode as a mask to form a second trench extending tothe first semiconductor region through the second and fourth to sixthsemiconductor regions.
 8. The method according to claim 1, wherein theconductive member is a metal.
 9. The method according to claim 8,wherein the metal is tungsten.
 10. The method according to claim 1,wherein the conductive member is a semiconductor having the firstconductivity type and a resistance lower than the second semiconductorregion.